This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-048641, filed Feb. 23, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a rotary anode type X-ray tube, particularly, to a rotary anode type X-ray tube that permits preventing the bearing portion of the rotating mechanism rotatably supporting the anode target from being abraded.
2. Description of the Related Art
The rotary anode type X-ray tube is known as an electron tube in which a rotating anode target is irradiated with an electron beam so as to permit the anode target to emit an X-ray and is used in, for example, a medical diagnostic apparatus.
FIG. 1 is a cross sectional view schematically showing a part of the conventional rotary anode type X-ray tube. As shown in the drawing, the conventional rotary anode type X-ray tube comprises an anode target 91 arranged within a vacuum envelope (not shown). The anode target 91 is fixed to a support shaft 92, and the support shaft 92 is joined to a rotating mechanism 93.
The rotating mechanism 93 includes a rotor section and a stator section. The rotor section includes an intermediate rotor 94 joined to the support shaft 92, an outside rotor 95 mounted to the outside surface of the intermediate rotor 94, and an inside rotor 96 mounted to the inside surface of the intermediate rotor 94. A stator 97 is inserted into the inner region of the inside rotor 96 so as to be coupled with the inside rotor 96. That opening of the inside rotor 96 which corresponds to the right side in FIG. 1 is sealed with a thrust ring 98, and the stator 97 extends through the thrust ring 98 to the outside. Dynamic slide bearings which is so called as liquid film bearings or metal lubricated bearings are formed between the inside rotor 96 constituting the rotor section of the rotating mechanism 93 and the stator 97 constituting the stator section of the rotating mechanism 93 and between the thrust ring 98 and the stator 97 noted above. To be more specific, a pair of spiral grooves are formed in each of two regions apart from each other in the axial direction of, for example, the stator 97, and a liquid metal lubricant is supplied into the spiral grooves and the peripheral regions thereof so as to form radial dynamic slide bearings 99A and 99B.
Also, a spiral groove of a herringbone pattern is formed on the edge surface of the stator 97 shown on the left side of FIG. 1. A liquid metal lubricant is also supplied into the spiral groove and the peripheral region thereof so as to form a thrust dynamic slide bearing 100A, 100B. Likewise, a spiral groove of a herringbone pattern is formed on the surface, which corresponds to the right side of FIG. 1, of the thrust ring 98 facing a stepped portion 97A of the stator 97, and a liquid metal lubricant is supplied into the spiral groove and the peripheral region thereof so as to form a thrust dynamic slide bearing 100B. Incidentally, the letter xe2x80x9cMxe2x80x9d shown in FIG. 1 denotes the tube axis.
In the rotary anode type X-ray tube of the construction described above, a dynamic slide bearing is used as a bearing of the rotating mechanism. However, a roller bearing such as a ball bearing is also used in the bearing of the rotating mechanism.
The dynamic slide bearing is not abraded during the rotation and, thus, burning is not generated in the bearing even where the dynamic slide bearing is rotated at a high speed. However, the rotor is brought into contact with the stator at the starting time of the rotation and when the rotation is stopped. Also, where the dynamic slide bearing is rotated at a low angular speed, the dynamic slide bearing performs its function under the state that the rotor is partly kept in contact with the stator so as to bring about a so-called xe2x80x9cmixed lubricating statexe2x80x9d in which the contact force and the dynamic pressure are exerted simultaneously. It follows that, if the load of the rotor is heavy, a large stress is imparted to the contact portion.
As a result, the bearing surface is abraded at the start-up time or when the rotation of the rotor is stopped. Alternatively, burning is generated, and the rotor and the stator are meshed with each other so as to bite each other in the contact portion, thereby lowering the durability. Also, where the rotor has a large moment of inertia, the energy that is generated when the rotation of the rotor is stopped is increased so as to lower the durability. Particularly, where the center of gravity of the rotating portion is positioned outside the bearing, the force applied to the contact portion is rendered greater than the actual load so as to give a detrimental effect to the durability.
It should also be noted that the dynamic slide bearing requires a large rotational torque at the start-up time of the rotation. It follows that a rotation starting apparatus generating a large torque is required so as to render the apparatus bulky and increase the power consumption. In addition, where the angular speed of rotation is increased, the heat generation derived from the shearing force of the liquid metal lubricant is increased so as to elevate the temperature of the liquid metal lubricant. If the temperature of the liquid metal lubricant is elevated, the viscosity of the liquid metal lubricant is lowered so as to shorten the life of the liquid metal lubricant.
On the other hand, the roller bearing is advantageous in that the construction of the bearing is simple so as to facilitate the manufacture of the bearing. Also, since the rotor is not brought into contact with the stator at the start-up time and when the rotation of the rotor is stopped, the abrasion of the rotor and the stator is prevented. However, the bearing surface is abraded under the state of a high speed rotation or under the state of a heavy load so as to bring about the cracking and increase the noise during the rotation, with the result that the rotational torque is increased so as to cause burning in the bearing and generate noises.
An object of the present invention is to provide a rotary anode type X-ray tube capable of suppressing the abrasion in the bearing portion of the rotating mechanism so as to assure good rotating characteristics over a long period of time.
According to a first aspect of the present invention, there is provided a rotary anode type X-ray tube, comprising:
an anode target configured to emit an X-ray;
a rotating mechanism configured to rotatably support the anode target, including a rotor section and a stator section;
a vacuum envelope configured to house the anode target and the rotating mechanism; and
a bearing mechanism arranged between the rotor section and the stator section and including a dynamic slide bearing using a liquid metal lubricant and roller bearings.
According to a second aspect of the present invention, there is provided a rotary anode type X-ray tube, comprising:
an anode target configured to emit an X-ray;
a rotating mechanism configured to rotatably support the anode target, including a first rotor section to which the anode target is coupled, a second rotor section, a dynamic slide bearing being arranged between the first rotor section and the second rotor section, and a stator section, roller bearings being arranged between the second rotor section and the stator section; and
a vacuum envelope configured to house the anode target and the rotating mechanism.
Further, according to a third aspect of the present invention, there is provided a rotary anode type X-ray tube, comprising:
an anode target configured to emit an X-ray;
a rotating mechanism configured to rotatably supporting the anode target, including a first rotor section to which the anode target is coupled, a second rotor section, roller bearings being arranged between the first rotor section and the second rotor section, and a stator section, a dynamic slide bearing using a liquid metal lubricant being arranged between the second rotor section and the stator section; and
a vacuum envelope configured to house the anode target and the rotating mechanism.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.